Laboratory studies indicate that tumor hypoxia may be an important prognostic factor in cancer radiotherapy and chemotherapy, but as yet there are no widely applicable techniques for assessing tumor hypoxia in humans. The lack of such a techniques makes the design and interpretation of clinical trials of "anti-hypoxic cell" therapies difficult. We have conducted preliminary studies which suggest that tumor hypoxia can be detected non-invasively and individually by using 19F MRS to measure the retention of hexafluoromisonidazole (CCI-103F) in tumors. We will use several mouse tumor models to determine whether CCI-103F retention in tumors, as measured by 19F MRS, can be used to assess radiobiological hypoxia under a wide range of conditions. We propose to correlate CCI-103F retention to measure tumor hypoxia after irradiation, we will able to assess reoxygenation in individual tumors. This is something that no other current technique can do, even in animal tumors. Although numerous authors have discussed the utility of 31P MRS in the study of tumor response to treatments, 31P MRS alone may not be a reliable method for predicting tumor radiosensitivity. We suggest that a combined 19F/31P method could provide information about tumor hypoxia and tumor response to treatment that would be more useful than the information provided by either technique alone. We hypothesize that a combined 19F/31P MRS method will provide integrated information in hypoxia, response to irradiation, and reoxygenation in individual tumors.